feat(rust): a Steam account linked on the site is a Rust link (D392, D396)

Registers core's MODULE_API 1.17.0 identity hooks (coreApi ^1.17.0):
onIdentityLinking refuses a Steam id that is another website account's Rust
link (D396); onIdentityLinked writes the Rust link with the PRIMARY KEY as the
race guard, reconciles Teams and raises the linked notice, as /link does.
Unlinking Steam leaves the Rust link (D393); the Rust account page says so.

ci/core-ref.json still pins the 1.15.0 core and must move to the website
branch head carrying 1.17.0 before CI can pass.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E14m6SuuY6i1vASFeGDBeY

engagement-triggers.json

@@ -1,6 +1,6 @@
 {
   "_comment": "Generated freeze of module-rust's engagement contract (docs/modules/rust/PLAN.md §25). Regenerate with `npm run engagement:manifest` in server/. A renamed variable, a changed type or a widened ceiling breaks stored templates and rules, so the diff here is the review signal.",
-  "coreApi": "^1.15.0",
+  "coreApi": "^1.17.0",
   "triggers": [
     {
       "id": "rust.base.destroyed",